J2CL

J2CL is a lightweight transpiler developed by Google that converts Java source code into highly optimized JavaScript designed to work seamlessly with the Closure Compiler. It allows developers to write applications in Java while targeting web environments, enabling strong type safety and code reuse across platforms. Unlike monolithic frameworks, J2CL focuses purely on transpilation, leaving optimization, bundling, and runtime concerns to the broader toolchain, which provides flexibility in modern development workflows. The generated JavaScript is fully type-safe and can be aggressively optimized by the Closure Compiler to reduce code size and improve performance. J2CL is widely used internally at Google in large-scale applications such as Gmail and Google Docs, demonstrating its production readiness and scalability. It supports interoperability between Java and JavaScript through tools like JsInterop, enabling seamless integration of code across languages.